1. Field of the Invention
This invention relates to an apparatus for repetitively cutting a continuously moving web into desired lengths.
The term web as used herein is intended to mean a flexible and relatively long web-like material whose thickness is from 5 .mu. to 5 mm and whose width is from 10 cm to 3 m, for example, such as plastic films comprised of polyvinyl chloride, polycarbonate, acrylonitrile-stylene copolymer, ABC resin, polyester, polyester resin containing glass fiber, cellulose inductives, etc., sheets such as paper, synthetic paper, etc., and thin metal plates such as aluminum, copper, etc.
2. Description of the Prior Art
A web is generally wound onto a roll during manufacture, and is thereafter unwound during working and cut into desired lengths in accordance with the intended uses thereof. To efficiently cut the web various devices have been used, one of which comprises a pair of rotary knife edges oppositely disposed above and below the web and adapted to be driven at the same peripheral speed as the web to thereby cut it into desired lengths by the engagement of the edges with each other. Another device used is a sliding carriage type of cutter, which comprises an upper knife edge movable up and down with respect to a lower knife edge, with the travelling web disposed therebetween. Both knife edges are simultaneously moved parallel to the web, and at the same time the upper edge is moved downwardly towards the lower edge to cut the web into desired lengths.
In the former device the knife edges are each mounted on individual, rotatable, drum-like supports, so the setting and adjustment of the knife edges for proper cutting engagement is time-consuming and difficult. In addition, the coincidence of the peripheral speeds of the knife edges with each other and with that of the web is difficult to achieve due to mechanical errors such as slip and back-lash in the rotatable supports and drive system for the web, which adversely affects the accuracy of the cut web lengths.
In the latter device the engagement setting of the knife edges and the drive speed control are more easily implemented, as a result of which the accuracy of the cut web lengths has been considerably enhanced. When the cut length error tolerance is less than .+-. 500 .mu., however, it is impossible to obtain sufficient accuracy with existing reciprocating carriage types of web cutters.
Investigations by the present inventors have revealed that when the drive system for moving the upper and lower knife edges parallel to the web and the drive system for moving the upper edge downwardly towards the lower edge operate with a slight time displacement relative to a common drive source, relatively large torque variations result which produce uneven rotation of the drive source. This causes a displacement in the point engagement between the knife edges, i.e. in the point where the web is cut, and such displacement error increases as the speed of the web increases. The drive source for the knife edges generally also serves as the drive source for the web to simplify construction and decrease manufacturing cost, and it is therefore difficult to change the cut length setting of the web and unsuitable for use in cutting webs whose lengths must vary.